1. Field of Invention
The invention relates to an optical device and, in particular, to an optical circulator with several optical ports installed on the same side.
2. Related Art
Optical circulators are a kind of optical passive device with at least three optical ports. Light entering a first optical port is output from a second optical port and light entering the second optical port is output from a third optical port. When there are more than three ports, light entering the i""th optical port is output from the (i+1)""th optical port. Therefore, the optical path inside the optical circulator is irreversible.
Different optical ports of most known optical circulators are not situated on the same axis. Polarizing bear splinters (PBSs) have to be used, as proposed in U.S. Pat. No. 5,878,176. They do not only have higher prices, but also larger sizes. To decrease the volume of the products, most people design all optical pores on the same axis. There are several means to implement this. For example, U.S. Pat. No. 5,921,422 uses a thermally expanded core (TEC) fiber. U.S. Pat. Nos. 5,973,823 and 6,049,427 can both effectively minimize the product volume by aligning optical ports on the same axis. To lower product prices and to facilitate product assemblies, U.S. Pat. No. 5,973, 823 utilizes the relative angle between a multi-layer Faraday spin crystal and a birefringent crystal optical axis so as to abandon the need for half wave plates. U.S. Pat. No. 6,002,512 employs a latchable Faraday spin crystal to decrease the number of half wave plates. U.S. Pat. Nos. 5,921,039 and 6,049,426 do not only have all optical ports on the same axis, but also need two-core fiber collimator among the three optical ports. U.S. Pat. Nos. 6,014,244; 6,014,475; and 6,088,491 insert one of several lenses among crystals to change the optical path. Nevertheless, the above-cited references have a general feature: the i""th optical port and the (i+1)""th optical port are on different ends of the optical circulator. Therefore, their optical circulator products have a longer length and require more crystals. U.S. Pat. Nos. 6,097,869 and 6,111,695 both use one reflective mirror to make all optical ports on the same side. However, the optical ports of U.S. Pat. No. 6,097,869 are composes of TEC fibers. Each optical port requires an extra convergent lens. U.S. Pat. No. 6,111,695 totally needs three birefringent crystals to achieve the circulation function, resulting in more length and cost.
An objective of the invention is to decrease the number of crystals needed in an optical circulator and the length of the optical circulator, thus providing an optical circulator with a small volume.
Another objective of the invention is to provide an optical circulator with all its optical ports situated on the same side.
The invention uses an optical reflective device so that a light beam entering through an optical port is reflected and passes through all optical devices (i.e., all optical crystals) on its optical path so as to be guided to the next optical port. Through such a design, all crystals can be repeatedly used to reduce the number of crystals needed and the length of the optical circulator.
The invention uses a miniaturized fiber collimator as the I/O port of the circulator. Aside from reducing the area of crystals and shortening the crystal lengths, it further has feature of an extremely good expandability. The invention uses a non-reciprocal reflector, therefore all optical ports of the optical circulator can be installed on the same side, simultaneously achieving the circulation function and the optical designs of no polarization dependent loss (PDL) and no polarization mode dispersion (PMD).
The invention uses a proper reciprocal-non-reciprocal optical crystal combination to generate a specific linear polarization direction to selectively generate light beam walk-off, satisfying the irreversibility property of the optical path within the optical circulator.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
The present invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein:
FIG. 1 schematically shows a basic structure of the optical circulator disclosed in the invention;
FIG. 2 is a first embodiment structure of the optical reflective device;
FIG. 3 is a second embodiment structure of the optical reflective device;
FIG. 4 is a third embodiment structure of the optical reflective device;
FIG. 5 is a fourth embodiment structure of the optical reflective device;
FIG. 6 shows the optical structure according to the first embodiment of the disclosed micro-reflective optical circulator;
FIGS. 7A through 7J show detailed crystal orientations and optical polarizations along the paths in the propagation direction of FIG. 6;
FIG. 8 shows the optical structure according to the second embodiment of the disclosed micro-reflective optical circulator;
FIGS. 9A through 9H show detailed crystal orientations and optical polarizations along the paths in the propagation direction of FIG. 8;
FIG. 10 shows the optical structure according to the third embodiment of the disclosed micro-reflective optical circulator;
FIGS. 11A through 11J schematically show the beam polarization direction of the first crystal structure in FIG. 10;
FIGS. 12A through 12H schematically show the beam polarization direction of the second crystal structure in FIG. 10;
FIG. 13 shows the optical structure according to the fourth embodiment of the disclosed micro-reflective optical circulator; and
FIGS. 14A through 14J schematically show the beam polarization direction of the crystal structure in FIG. 13.